This invention relates to an air flow rate measuring device which measures a flow rate of air taken into an internal combustion engine and more particularly to an air flow rate measuring device which is provided with a hot wire installed in the by-pass passage communicating with the main air passage.
In a conventional device the flow rate of air taken in is measured by installing a wire in a main air passage, supplying electric current to the wire to compensate for the amount of heat lost in the main air stream, and measuring the voltage drop across the wire caused by the current flowing through it. This kind of device, however, has the disadvantage that the wire is easily fouled with dust contained in the suction air. Furthermore, when the engine backfires, hot air from the engine will not only foul the wire but may damage it.
To overcome these drawbacks, it has been a practice to install the hot wire in the by-pass passage which communicates at both ends with the main air passage.
In FIG. 2 of commonly assigned co-pending U.S. patent application Ser. No. 28,889, a by-pass passage installed with a flow rate sensor, such as a wire, is provided to by-pass the venturi of the main air passage. An inlet of the by-pass passage is formed in the upper portion of the venturi. An annular outlet that is, the lower end of the by-pass passage opens into the main air passage through the venturi.
With this type of the intake air flow rate measurement device, the sensor or hot wire will not be damaged if the backfiring occurs because the hot air from the engine flows back through the main air passage not the by-pass passage. However, since the inlet of the by-pass passage is formed in a portion on the circumference of the venturi and opens upward, if there is not a cover covering the inlet, the dust may fall into the by-pass passage and adhere to the wire and the blast from the backfire may enter it through the inlet, fouling the wire. Although the degree to which the wire of this device will be fouled or damaged may be smaller than that for the type with the wire installed in the main air passage, it still cannot be protected completely from being fouled. If the wire is fouled, the constants of the material and shape of the wire (which is described later) will change, deteriorating the accuracy of the measurement of intake air flow rate.
The member disposed to cover over the inlet of the by-pass passage will alleviate the drawback to some extent, but may also reduce the flow rate of air taken into the by-pass to some extent, whereby the accuracy of the measurement of the intake flow rate may be reduced.